The present invention relates to a computed tomography (CT) system. Specifically, the present invention relates to a CT system in which a radiation source such as an X-ray source irradiates radiation rays to an object such as a patient while spirally moving the radiation source relative to the object, the radiation rays transmitted through the object are detected with a detector, measuring projection data is obtained from the signal detected with the detector, and tomographic image data of the object is obtained from the measuring projection data.
With a conventional CT system, a tomogram at a desired slice plane of an object, e.g., patient, is taken in the following manner. X-rays are irradiated from an X-ray source to a stationary patient while rotating the X-ray source about the patient in the vertical plane containing the slice plane. Projection data from a plurality of directions at the slice plane is collected, and a tomographic image is reconstructed using the projection data. For the convenience of description, such projection data is called stationary patient (object) projection data.
If a plurality of tomograms at a plurality of different slice planes of a patient are to be obtained using such a conventional CT system, the following operations become necessary. After collecting projection data while rotating an X-ray source within one slice plane, the X-ray source temporarily stops its operation. During this stop period, the patient is moved horizontally to enable the X-ray source to be positioned within the vertical plane containing the next slice plane. Then, projection data for this slice plane is collected. The conventional CT system requires such operations, resulting in a long period of restricting the patient and a poor work efficiency of the system.
To eliminate such disadvantages, a CT system providing spiral scanning has been proposed. Such CT systems are disclosed in U.S. Pat. No. 4,630,202 corresponding to JP-A-59-111738, and U.S. Pat. No. 4,789,929 corresponding to JP-A-62-87137 and JP-A-62-139630.
CT systems disclosed in U.S. Pat. Nos. 4,630,202 and 4,789,929 process measuring projection data obtained through spiral scanning by the methods specific to the systems, and obtain data corresponding to the stationary patient projection data. The obtained data is assumed to be the stationary patient projection data, and a tomographic image is reconstructed in the manner similar to a conventional CT system of a stationary patient type.